Steam reforming processes include pre-reforming, tubular reforming, heat exchange reforming, catalytic partial oxidation (CPO), auto-thermal reforming and secondary reforming. In the temperature range 400-1300° C. and in the presence of a metal-based catalyst (nickel), steam reacts with hydrocarbon feedstocks such as natural gas and naptha to yield syngas (carbon monoxide, carbon dioxide and hydrogen). Steam reforming catalysts usually comprise nickel on a refractory support material. The nickel is present as nickel oxide, which is reduced prior to use in the steam reformer.
“Coking” is a common phenomenon in steam reforming processes. It refers to formation of carbon, such as pyrolytic, encapsulating, or whisker coke, on metal surfaces. The ability to suppress the level of coking may be particularly advantageous for catalytic processes such as steam reforming. For example, coke formation may damage the mechanical structure of a catalyst in high-temperature applications (e.g. tubular reforming, heat exchange reforming), as well as reduce the steam reforming activity of the catalyst.
It is known that alkali metals (those in Group I of the periodic table) promote gasification of carbon, enhance adsorption of water and retards dissociation of methane and higher hydrocarbons on nickel catalysts in steam reforming processes.
In particular, alkali metals can increase the resistance of the catalyst to coking. For example, U.S. Pat. No. 7,378,389, U.S. Pat. No. 5,773,589, and Brazilian patent application no. PI 1000 656-7 describe the impregnation of support materials with alkali metal salts. Such processes require a two-step impregnation (impregnation with alkali metal salts and impregnation with metal salts), and the alkali metal is not an integral part of the support material.
Similarly, US 20012/0070367 also discloses a catalyst support containing Al, Mg with subsequent addition of potassium, thereby merely impregnating it onto the support and not becoming an integral part of it.
However, alkali metals have a tendency to migrate from the supported nickel catalyst. This can cause problems down-stream from the steam reforming process, but also leads to loss of the useful properties of the alkali metal on the catalyst.
Other patent publications concerning steam reforming catalysts include WO 2012/031341, U.S. Pat. No. 7,767,619 and U.S. Pat. No. 6,984,371. In particular, publications where alkali is on the support yet not an integral part of it, include WO 2012/121761, US 2006/0008413, GB 1,003,702, U.S. Pat. No. 2,847,836, GB 1,166,491 and GB 1,509,557.
The need remains for a supported nickel catalyst in which coking is reduced for an extended period, i.e. one in which the migration of alkali metal from the catalyst is reduced.